Somatic embryos are embryos formed in tissue cultures of plants, although there are reports of somatic embryos forming in vivo. The structure and morphology of somatic embryos is like that of zygotic embryos from true seed, that is somatic embryos go through the same development stages--proembryo, globular, heat-shaped and torpedo-shaped morphology. However, somatic embryos lack a seed coat, and also sometimes lack a suspensor. Unless they are specially treated, somatic embryos also lack the dormancy that characterizes true seed (Gray et al., 1995a).
Somatic embryos are clonal in origin and thus multiplication using somatic embryos can have the potential for exceedingly high rates of vegetative increase and is therefore of considerable commercial interest. Somatic embryos also lend themselves to large scale production in bioreactors when grown in liquid culture (Bajaj, 1995a) and this form of propagation is of commercial interest when somatic embryos are transformed to provide novel plant forms.
Regeneration via somatic embryogenesis is an attractive option for plant tissue culture. Somatic embryos reportedly provide more stable regenerants than shoots. Another advantage of regeneration systems using somatic embryos is their apparent single cell origin. This means that it is unlikely that regenerants are of chimerical origin, since, if a regenerant originates from a cluster of cells rather than a single sell, the plant tissues may be chimerical or unstable and produce off-types. The availability of somatic embryogenesis protocols for potato, or other crop species recalcitrant to somatic embryogenesis, will permit these crops to take advantage of any new artificial seed technology advances.
Immature somatic embryos are ideal material for transformation protocols (Ellis, 1995). The induction of somatic embryos from cultured tissues of Vitus vinifera (Goussard and Wiid, 1993) and Citrus (D'Onghia et al., 1979) has led to the elimination of several viruses and viroids. Scorza et al. (1996) disclose the transformation of somatic embryos of `Thompson Seedless` grape. The use of somatic embryo technology for potato and other species recalcitrant to somatic embryogenesis, would open new avenues for research and development. As somatic embryos are reported to be more stable regenerants than adventitious shoots, the use of somatic embryos as transformant tissues may overcome the difficulties with off-types which are inherent with the use of stem internode leaf and microtuber tissue. Somatic embryos are suitable for transformation via Agrobacterium tumefaciens (Mathews et al., 1992), microinjection (Neuhaus et al., 1987) and particle bombardment (Wilde et al., 1992).
Many plant species regenerate shoots from tissues cultured in vitro, but the low frequency of shoot regeneration and the lack of stability (trueness-to-type) makes this technology less desirable. This is to be contrasted with the proposed somatic embryo technology described herein. This technology is very productive and results in many embryos being regenerated from one explant. Furthermore, this technology can be readily adapted for use with existing or future transformation technologies. Prior to transforming plant tissues with novel genes, a regeneration technology has to be in place or there is no way to recover transformed plants. Plant tissues regenerated as shoots or somatic embryos in vitro can be used to carry valuable genetic material coding for the production of vaccines, disease resistant plants, superior agronomic characteristics, food processing qualities, etc.
Somatic embryos can be cryopreserved using liquid nitrogen without loss of viability. Cryopreservation is an efficient means of maintaining germplasm and enables plant material to be transported over large distances. Furthermore, somatic embryos are suitable for the development of artificial seed technology (U.S. Pat. No. 5,572,827; Bajaj, 1995 a). Regeneration technologies in which somatic embryos are the propagule can produce many more regenerants and lead the way for the production of artificial seed (synseed) technology. Synthetic seed, appropriately coated with products providing nutrients and protection against desiccation and microorganisms may be used within fluid drilling technology to "sow" the synseeds under field conditions.
Somatic embryos have been produced from juvenile somatic tissues, or from cultured stocks of juvenile tissues, for a few potato cultivars (Bragd.o slashed.-Aas, 1977; Garcia and Martinez, 1995; Pretova and Dedicova, 1990). Pretova and Dedicova (1992) disclose the production of cotyledonary somatic embryos obtained from zygotic embryo tissues (i.e. juvenile tissues) from potato in media using BAP, dicamba or 2,4-D. They note that friable callus is obtained after 2 weeks. However, a granular callus is not obtained for 6-8 weeks on media containing 2,4-D. After subculturing this callus onto hormone free media supplemented with sucrose, somatic embryos appear within 14-18 days. Bragd.o slashed.-Aas (1977) discloses the formation of callus from potato tuber tissue and the regeneration of plants from this callus tissue. It is noted that there is great variety in the production of callus depending upon the genetic background of the material used. Furthermore, the applicability of this method is limited to certain clonal sources and therefore lacks applicability to a range of cultivars . Hutchinson et al. (1996) teach the production of somatic embryos using explants obtained from six day old etiolated hypocotyls of geranium. The media contained either TZD, or IAA+BAP, and embryo formation was observed over a 21 day period. The production of embryos was not investigated past the 21 day period.
Garcia and Martinez (1995) disclose the induction of somatic embryos from cultured stem sections of the potato variety `Bintje`. The protocol employed by these workers requires about 20 to 25 weeks to produce somatic embryos, and another 5 to 6 weeks to produce plantlets from these somatic embryos. However, the method of this invention produces somatic embryos after 4 to 6 weeks, with plantlets forming within 2 to 4 weeks from these somatic embryos. Furthermore, within the prior art, somatic embryos from potato have only been obtained from tetraploid genotypes, however, following the protocol of this invention, somatic embryos have been produced using tetraploid, diploid and monoploid potato cultivars. There are also many other advantages associated with the production of somatic embryos following the method disclosed herein (e.g. see Table 1).
Similarly, somatic embryos from seedling tissues, such as cotyledons, which are juvenile in a physiological sense, of tomato (Chen and Adachi, 1994; Gill et al., 1995) and lettuce (Zhou et al., 1992) have been prepared. Furthermore, somatic embryos have been prepared using hypocotyl sections of juvenile tomato seedlings. However, the ability to produce somatic embryos from mature tissues is in general lacking. This is also true with conifer somatic embryo technology as well, which is directed to juvenile (zygotic embryo) tissues. The preparation of somatic embryos using zygotic (seed) embryos is a common way of achieving embryogenesis. However, this system has the disadvantage that the variability of a heterozygous species produces unreliable propagules for industrial applications.
There is a real need within the art to be able to propagate a plant that has already been selected for a set of desirable traits. Current breeding practices typically require sexual crossing, grafting or other procedures that results in the offspring never being identical to the parent. Since the ability to propagate the exact same individual does not exist with these techniques, a breeder must then rescreen the offspring in order to reselect and validate the stock material for the desired set of characteristics. The ability to propagate a plant comprising the same set of characteristics as the parent, is available through clonal propagation. However, the use of juvenile tissues for the production of somatic embryos, does not improve this situation as the material used for somatic embryogenesis has itself not been subject to the screening process. The ability to produce somatic embryos from mature tissues, however, provides the ability to generate clonal propagules from an individual plant that has been subject to a screening process, so that the propagule comprised the same set of characteristics as the pre-selected parent.
The regeneration of tissues from mature plants of potato, tomato and lettuce via somatic embryogenesis would enable industry to rapidly propagate elite plants of horticultural value. By contrast, propagating tissues from zygotic embryos is not feasible in heterozygous plants such as potato which are vegetatively propagated because of the variability of seedling material. Furthermore, somatic embryos lend themselves to transformation of valuable genotypes.
By using the method of the present invention:
somatic embryos have been obtained on a plurality of potato (Solanum tuberosum L.) cultivars, of a variety of ploidy levels in vitro. This protocol can be used for clonal multiplication of selected potato genotypes, regeneration after gene transformation or bombardment, large-scale production of somatic embryos for artificial seed production, etc.; PA1 somatic embryos are obtained within significantly shorter time frames than prior art methods; PA1 somatic embryos can be induced to form on various somatic tissues of the potato, including stem internodes, roots, leaves, microtuber slices; PA1 somatic embryos can be produced on stem internode tissue from plantlets in vitro of tomato; PA1 Somatic embryos have been produced on seedling leaf and stem tissue of lettuce in vitro. Somatic Embryos have also been produced on mature tissues of lettuce in vitro. This is of commercial interest because superior genotypes can be multiplied clonally. PA1 the stock tissue culture plantlets are maintained on a medium (Medium #1) comprising: PA1 the proliferation medium, medium #2, comprises: PA1 and further comprises the constituents selected from the group consisting of; PA1 and the medium comprising at least two growth regulators, medium #3, comprises: PA1 obtaining somatic embryos within shorter time frames than prior art methods (weeks to months shorter than previously published methods); PA1 adaptability to a range a wide variety of plant species, and cultivars that typically have not been amenable to the production of somatic embryos; PA1 effectiveness in obtaining somatic embryos from both mature or juvenile tissues. Prior art methods are typically limited to juvenile tissues; PA1 with regards to potato cultivars, somatic embryos have been produced using tetraploid, diploid and monoploid tissues. All prior art methods have only been successful using tetraploid tissues, and, to the best of our knowledge, no somatic embryos have previously been reported using diploid or monoploid potato tissues. As a results of these advantages, mature plants may be selected for desirable properties and clonally propagated to ensure maintenance of the genotype and phenotype of the stock tissues. This last advantage has not been previously available within this field since prior art methods for obtaining somatic embryos require juvenile tissues. Any selection of plants with desirable traits, therefore had to place using plants obtained from somatic embryos. This requires producing large amounts of somatic embryos from which plants are regenerated, and the screening of these libraries of regenerated plant stocks.